During the smelting of aluminum scrap, such as used beverage containers and the like, large quantities of salt flux, normally containing sodium and/or potassium chlorides, are added to the smelter furnaces. This flux accumulates on top of the molten bath and forms a dross which typically contains aluminum values in the form of metallic aluminum, as well as aluminum oxides and metal salts.
There are various known procedures for processing secondary aluminum dross or salt cake. One such process involves initially reducing the particle size of the dross, e.g., by impact and ball milling processing. Following particle size reduction, the dross is melted under a salt flux cover to dissolve the contaminants therein, mostly aluminum oxide, and recover the aluminum metal. Typically, rich dross (containing 30-70% aluminum) and low-grade dross (containing 5-30% aluminum) are melted in rotary furnaces using a sodium chloride-potassium chloride flux which may include 2-5% cryolite. As the flux becomes contaminated with aluminum oxide, it is removed as salt slag (by skimming or raking) and then solidified. Although this salt slag contains potentially valuable materials that could be recovered, it is generally the practice to dispose of the salt slag in a landfill. However, the soluble salts present in the slag represent a potential source of pollution to surface and ground water supplies. Accordingly, it is increasingly important, both for economic as well as environmental reasons, to eliminate landfill disposal of these salt slags.
Other previously employed procedures for processing secondary aluminum dross involve crushing the aluminum dross, separating the larger and smaller particles, and concurrently treating the larger and smaller particles in distinct leaching operations. In a specific prior process of this general type, magnesium chloride is added to the leaching solution, which is used to leach the larger particles, to maintain the pH of that solution in the range of about 7-8.5. This process is described in co-pending and commonly owned U.S. patent application Ser. No. 07/756,359, filed on Sep. 9, 1991, the specification of which is hereby incorporated herein by reference.
Although these known prior procedures are intended to remove the metal salts from the aluminum dross, they may not achieve maximum salt removal from the aluminum dross, and thus the residual solids may not be sufficiently inert for landfill disposal.